Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in surface coating materials as a vehicle or polymeric binder to advantageously provide toughness, flexibility, adhesion, and chemical resistance to the applied coating film. Hence, water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity and must not alter the taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by adsorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof.
In commonly assigned U.S. Pat. No. 4,212,781, a process is disclosed for modifying epoxy resin by reacting the epoxy resin with addition polymerizable ethylenic monomer in the presence of at least 3% by weight of benzoyl peroxide (or the free radical initiating equivalent thereof) based on monomer at a suitable reaction temperature to produce a reaction mixture comprising an epoxy-acrylic copolymer mixture containing epoxy resin, graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. The in-situ polymerized monomers include acid functional monomers to provide acid functionality in the reaction mixture sufficiently high to effect stable dispersion of the resulting reaction product in a basic aqueous medium. Similarly, commonly assigned U.S. Pat. No. 4,522,961 pertains to a mixture of polymers comprising a self-curing emulsion polymer (latex), an epoxy-acrylic graft copolymer, and preferably a phosphate additive. Related commonly assigned patents are U.S. Pat. Nos. 4,285,847 and 4,399,241, and 5,212,241. Most prior art water-dispersed epoxy coatings utilize relatively high levels of organic solvent to assist processing of the epoxy resin. Although epoxy containing coatings have long been the standard of excellence in beverage can liners, such coatings cannot be prepared without significant amounts of solvent, where typically 50% to 100% volatile organic solvent is required based on solids (about 2.5 to 4 lb./gal.). Recent environmental concerns and legislative pressure have created the need for a zero or near zero VOC can coating. Furthermore, such coatings require environmentally undesirable external crosslinkers such as melamine/formaldehyde crosslinkers.
Polyesters containing copolymerized aromatic acid in the backbone provide good barrier films in beverage and food containers. Although carboxyl functional polyesters can be dispersed into water, low molecular weight polyesters having molecular weights below about 3,000 have limited colloidal stability as well as poor film properties, and hence, severely limit the potential of such coatings. Higher molecular weight polyesters exhibit better film properties but have very poor aqueous stability.
It now has been found that the use of small amounts of a low molecular weight, carboxyl functional addition copolymer as a polymeric dispersant substantially increases the aqueous stability of water dispersed polyester polymers and further enables emulsion crosslinking to produce low or zero VOC microgel particles having very high molecular weight but a particle size below about 0.15 micron. In accordance with this invention, very low or zero VOC polyester polymer particles having high molecular weight can be stably dispersed by using emulsion polymerization and by crosslinking the polyester with difunctional epoxy to produce crosslinked microgel particles. The final crosslinked particle size is controlled by composition, molecular weight, concentration, temperature, ionic strength and the relative amounts of polymeric dispersant, polyester, and microgel crosslinking. Particle size need not be controlled by mechanical shear and consequently is much easier to control and reproduce by maintaining reaction parameters constant. Multifunctional epoxy resin is added to the aqueous dispersion and reacted with carboxylic acid functionality to produce crosslinked microgel particles. The viscosity of the aqueous dispersion can be controlled by the amount of epoxy added, the composition and amount of acrylic dispersant, and the composition and amount of polyester. The resulting crosslinked microgel particles are typically less than about 0.15 microns and are stably dispersed in water without additional surfactants.
This invention incorporates the advantages of epoxy chemistry providing good barrier properties and excellent resistance to flavor absorption along with a synthesis technique that substantially reduces the level of organic solvents, organic amines and surfactants. The zero VOC aqueous coatings have extended storage stability along with controllable viscosity. Baked paint films utilizing the resulting polymeric binder are clear, glossy, solvent resistance, and water resistant. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.